upipe_avcenc: fix/enhance

- flush encoder when closing context
- properly unblock sink
upipe_avcenc: fix/enhance

- flush encoder when closing context
- properly unblock sink
tests: libav/sws and valgrind

- add real avcodec test
- add valgrind/osx posix_memalign suppression
- run sws_test/avcodec_test by default
fix distcheck